Sen. Charles Grassley, chairman of the Senate Judiciary Committee, asked FBI Director James Comey to provide more information about his decision to reopen the investigation into Hillary Clinton's emails, the ongoing probe of the Clinton Foundation and the Justice Department's reported efforts to block the bureau's progress.

"Without additional context, your disclosure is not fair to Congress, the American people, or Secretary Clinton," Grassley wrote in a letter Monday.

"The factual context is important," he added. "In addition, it is critical for the public to know whether the FBI has requested from the Justice Department vital investigative tools such as grand jury subpoenas and search warrants and whether it has been denied access to them."

That investigation has lasted more than a year and required the assistance of at least four FBI field offices around the country. In February, senior Justice Department officials reportedly pushed the FBI to abandon the foundation probe.

"If the FBI is denied the ability to gather evidence through compulsory means, Secretary Clinton and her aides have enormous leverage to negotiate extraordinary concessions in exchange for voluntary cooperation," Grassley wrote.

He cited the immunity agreements issued in the year-long Clinton email investigation as an example.

Two of Clinton's former aides and current lawyers, Cheryl Mills and Heather Samuelson, negotiated expansive production immunity deals that limited the FBI's ability to use emails found on laptops secured through the agreements.

Agents working on the separate Clinton Foundation probe could not use potentially relevant emails from those laptops due to the strict immunity protections, Grassley noted.

Those emails, which could number in the thousands, were discovered through an unrelated investigation into allegations that disgraced former congressman Anthony Weiner sent sexually-charged messages to a minor. Weiner is married to Huma Abedin, Clinton's former deputy chief of staff. Although they are estranged, the couple shared at least one laptop that contained the documents presently under investigation at the FBI.